Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light low molecular weight feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and, to a lesser extent, by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Crude oil, as produced from the reservoir, is typically accompanied by some volume of saltwater and particulate matter, also known as sediment or mud, from the reservoir formation. As used herein, the term “particulate matter” includes mud, mud blends, mud particles, sediment and other particles included in the hydrocarbon feedstock. Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency, e.g., viscosity, from water-like to tar-like solids, and in color from clear to black. A typical crude oil can contain about 84% carbon, 14% hydrogen, 1%-3% sulfur, and less than 1% each of nitrogen, oxygen, and even lesser amounts of metals, and dissolved salts. Refinery crude base stocks usually consist of mixtures of two or more different crude oils.
Field separation is used to remove the bulk of the saltwater and particulate matter, but some small quantity typically remains in the crude and is reported as basic sediment and water (BS&W) in reporting crude oil quality. Undesalted crude is sometimes processed in a refinery atmospheric pipestill in which the salt and particulate matter will concentrate in the bottoms fraction (atmospheric residue) from distillation of the crude. Additionally, crude or undesalted atmospheric residue can be further contaminated with salt prior to processing by contact with sea water during shipping. Prior to refining, the crude oil, or a bottoms fraction from distillation of the crude oil, is generally passed through a desalter which uses heat, clean water, and an electric current to break the emulsion, thereby releasing water and particulate matter from the suspension or emulsion with the crude oil or bottoms fraction. The salt and some of the particulate matter leave with the desalter effluent water. Some of the particulate matter remains on the bottom of the desalter vessel and is periodically cleaned out. The desalted crude or residue fraction derived from crude leaving the desalter is very low in salt and particulate matter. Highly effective desalters which employ electric current can typically remove more than about 90% of the salts present in raw crude.
In a situation where crude oil, atmospheric residue, or any other hydrocarbon feedstock containing salt and/or particulate matter is used as the feedstock for a reactor, a conventional desalter employing an electrostatic field would constitute a significant additional facility investment. Using undesalted crude oil or undesalted atmospheric residue as a feedstock in a conventional cracking furnace would, however, result in deposition of salt (primarily NaCl) and particulate matter as the liquid hydrocarbon feedstock was vaporized for cracking. Any non-volatile hydrocarbons would cause rapid coking around the dry point. The salt and particulate matter which also lay down causes corrosion and fouling of the convection tubes. Moreover, any salt remaining in the feed after the dry point and deposited in the radiant section of the furnace would result in removal of the protective oxide layer on the radiant tubes. Therefore, provisions must be taken to remove salt and particulate matter, to an extent sufficient to prevent damage in the furnace.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks, which contain a large fraction of volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil, and atmospheric residue. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.) otherwise known as asphaltenes, bitumen, or resid. The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the dry point where the lighter components have fully vaporized.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 450 and 1100° F. (230 and 590° C.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
U.S. patent application Ser. No. 10/188,461, filed Jul. 3, 2002, which is incorporated herein by reference, describes a process for cracking heavy hydrocarbon feedstock which mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins. The amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed, the pressure of the flash, the flow rate of the mixture stream, and/or the excess oxygen in the flue gas of the furnace.
U.S. patent application Ser. No. 10/975,703, filed Oct. 28, 2004, which is incorporated herein by reference, describes a process for cracking heavy hydrocarbon feedstock which mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, which uses an undesalted hydrocarbon feed to the convection section of a steam cracking furnace, and effects desalting downstream of the furnace inlet in a flash drum treating preheated feed.
While the references address the use of heavier hydrocarbon feedstocks, none of the references address the possibility of using a partially undesalted hydrocarbon feedstock for a cracking furnace. It has now surprisingly been found that it is possible to operate a steam cracking furnace with a hydrocarbon feedstock containing salt and/or particulate matter. This is particularly advantageous when the feedstock additionally contains non-volatile components.